As a hydraulic circuit arrangement for driving a hydraulic motor, there is known a circuit arrangement such as shown in FIG. 1.
In this circuit arrangement, a drain passage la of a hydraulic pump 1 is connected to first and second main circuits 3 and 4 through an operation valve 2, the first and second main circuits 3 and 4 are in turn connected respectively to first and second ports 6.sub.1 and 6.sub.2 of a hydraulic motor 5, and a supply of pressure oil to the first and second main circuits 3 and 4 is controlled by means of the operation valve 2. Furthermore, a counter balance valve 7 is disposed between the first and second main circuits 3 and 4. According to such arrangement, when the operation valve 2 takes its neutral position N, the first and second main circuits 3 and 4 are communicated with each other and the pressure oil flows out therefrom towards a tank 9, and accordingly, the counter balance valve 7 takes its neutral position N and check valves 8, 8 of the first and second main circuits 3 and 4 are closed to thereby block the hydraulic motor 5 side of the hydraulic circuit arrangement to prevent the hydraulic circuit arrangement to prevent the hydraulic motor 5 from being reversely rotated by an external force. On the other hand, when the operation valve 2 is shifted to a first position a or second position b, the pressure oil is supplied to the first or second main circuit 3 or 4, and the check valve 8 thereof is opened to thereby drive the hydraulic motor 5, and at the same time, the counter balance valve 7 is switched to a first position A or second position B by the pressure oil from the first main circuit 3 or second main circuit 4, and the hydraulic oil in the second main circuit 4 or first main circuit 3 is returned to the tank 9 through the counter balance valve 7 and the operation valve 2.
As mentioned above, the counter balance valve 7 provided for such hydraulic circuit arrangement for driving the hydraulic motor is switched in its position to the first position A or second position B by the pressure oil from the first main circuit 3 or second main circuit 4, and when all the pressure oil flows out from these main circuits 3 and 4, the counter balance valve 7 returns to its neutral position N.
Incidentally, when the supply of the pressure oil to the hydraulic motor 5 is stopped for stopping the operation thereof, the hydraulic motor 5 is rotated by the inertia of the travelling vehicle to attain a pumping function.
For this reason, at the time of stopping the hydraulic motor 5 by shifting the operation valve 2 to its neutral position N, when the counter balance valve 7 is shifted to its neutral position N and the check valve 8 is closed, the pressure oil in one of the first and second main circuits 3 and 4, disposed downstream of the check valve 8, exhibits a high pressure, thereby imparting a large shock at the time of stopping the operation of the hydraulic motor.
Such shock imparted at the time of stopping the operation of the hydraulic motor 5 can be reduced by delaying the shifting speed of the counter balance valve from the first or second position A or B to the neutral position N and throttling the flow of the pressure oil by the counter balance valve 7 so as to cause the pressure oil to gradually flow out the pressure oil to the tank 9. For example, as shown in FIG. 1, throttles 11, 11 are arranged in circuits 10, 10 connecting the counter balance valve 7 to the first and second main circuits 3 and 4, and the degree of throttling of these throttles 11, 11 is made small to thereby delay the returning speed of the counter balance valve 7 to the neutral position N from the first or second position.
However, in such arrangement, it will take considerable time for the counter balance valve 7 to return to its neutral position N, resulting in an occurrence of a cavitation or elongation of the time required for stopping the operation of the hydraulic motor, thereby creating a problem.
There is known a counter balance valve for eliminating this problem such as disclosed in Japanese Utility Model Laid-open Publication No. HEI 4-138103.
This is shown in FIG. 2, in which a valve body 20 is formed with a valve bore 21 to which first and second pump-side ports 22 and 23 and first and second motor-side, ports 24 and 25 are formed, a spool 26 is fitted into the valve bore 21 so as to be slidable in the longitudinal direction thereof to thereby establish communication between the first and second pump-side ports 22 and 23 and the first and second motor-side ports 24 and 25 or to block communication therebetween, and left and right pressure receiving chambers 28 and 29 are formed at positions between the left and right ends of the spool 26 and plugs 40 screwed to the end portions of the valve bore 21, respectively. The spool 26 is maintained in its neutral position, by left and right springs 27, 27 described hereinafter, at which the respective ports are closed, the spool 26 is shifted to a first travelling position by the pressure oil in the left pressure receiving chamber 28 at which the second pump-side port 23 is communicated with the second motor-side port 25, and the spool 26 is shifted to a second travelling position by the pressure oil in the right pressure receiving chamber 29 at which the first pump-side port 22 is communicated with the first motor-side port 24.
Furthermore, the spool 26 is formed with left and right small diameter portions 34 and 35 at its central portion and with axial bores 30 at its left and right end portions, the axial bores 30 at its left and right end portions, the axial bores 30 being communicated with the left and right small diameter portions 34 and 35 through small diameter bores 33, respectively. Further, pistons 31 formed with flanged portions 39 at outer peripheral portions thereof are inserted into the axial bores 30, respectively, and the springs 27 are disposed between the flanged portions 39 and the plugs 40 to keep the spool 26 in its neutral position N by the urging forces of the springs 27 through the flanged portion 39. Still further, the pistons 31 are respectively formed with axial oil bores 36, second small diameter bores 37 communicating the oil bores 36 with the left and right pressure receiving chambers 28 and 29, and radial bores 38 for opening the oil bores 36 to the outer peripheral surfaces of the pistons 31.
In the counter balance valve of the above structure, when the spool 26 takes its neutral position as shown in FIG. 2, the bores 38 are closed by the inner peripheral surfaces of the axial bores 30; when the spool 26 is shifted leftward or rightward from the neutral position to an intermediate position by a predetermined distance l.sub.2, the bores 38 are still closed by the inner peripheral surfaces of the axial bores 30; and when the spool 26 takes a position further shifted leftward or rightward by a predetermined distance l.sub.1, the bores 38 are communicated with the left or right pressure receiving chambers 28 or 29, and the first or second pump-side port 22 or 23 is communicated with the first or second port 24 or 25.
According to the counter balance valve of the structure described above, when a travelling vehicle is run by positioning the operation valve 2 at the first position a as shown in FIG. 1, the spool 26 is slid rightward by a distance l.sub.2 +l.sub.1 to take the travelling position. Then, when the operation valve 2 is shifted to its neutral position N from this position, the pressure oil in the first main circuit 3 flows out towards the tank 9 and the pressure therein is reduced, so that the spool 26 is slid leftward by means of the right spring 27.
At this time, the pressure oil in the left pressure receiving chamber 28 flows to the oil bore 36 through the second small diameter bore 37 and the bores 38, flows to the first pump-side port 22 through the first small diameter bore 33, and then flows out into the tank 9 through the first main circuit 3. Accordingly, since the pressure oil flow from the left pressure receiving chamber 28 is throttled only by the first small diameter bore 33, the pressure oil in the left pressure receiving chamber 28 smoothly flows into the tank 9, and as a result, since the spool 26 can be slid at high speed, the occurrence of any cavitation can be prevented and the speed reduction can be done with an improved follow-up performance.
At a time when the spool 26 is shifted leftward by the distance l.sub.1 to take its intermediate position, the bore 38 is closed by the inner peripheral surface of the axial bore 30, the left pressure receiving chamber 28 is communicated with the first pump-side port 22 through the second small diameter bore 37 and the first small diameter bore 33, the pressure oil flow is throttled by two small diameter bores 37 and 33 to attain substantially the same function as that attained by making a throttle diameter smaller than before, the pressure oil in the left pressure receiving chamber 28 hence flows out gradually towards the tank 9 and the spool 26 is moved leftward by the distance l.sub.2 to take the neutral position shown in FIG. 2. Accordingly, at this time, since the spool 26 is slid leftward with a low speed, a shock to be caused at the time of the operation stop of the hydraulic motor 5 is made small. Further, at a time when a vehicle runs down a slope, although substantially the same state is provided, a hunching phenomenon can be suppressed because of a large damping effect at this time.
As mentioned above, when the spool 26 of the counter balance valve 7 is slid from the travelling position to the neutral position, the spool 26 is slid with a high speed during a first half sliding time from the travelling position to the intermediate position and slide with a low speed during a latter half sliding time from the intermediate position to the neutral position, so that the occurrence of cavitation during the first half sliding time in which the counter balance valve 7 is slid with a high speed can be prevented and the operation of the hydraulic motor can be slowed and stopped with no occurrence of a large shock during the latter half sliding time in which the counter balance valve is slid with a low speed.
Therefore, according to the counter balance valve of the structure described above, the hydraulic motor can be reduced in speed and then stopped with reduced shock while preventing cavitation from occurring, and the hydraulic motor can be stopped in a short time by shifting the spool 26 to the neutral position in a short time.
However, because such counter balance valve is composed of the valve body 20, the spool 26 and the two pistons 31, the number of the constructional parts or components is increased, thereby increasing manufacturing costs and involving troublesome assembly work, thus also creating a problem.
Furthermore, since the piston 31 is provided with the flanged piston 39, the oil bore 36, the second small diameter bore 37 and the bore 38, the manufacturing of the piston 39 involves troublesome work and high cost, and accordingly, the counter balance valve itself becomes expensive.
The present invention was conceived, in view of the above problems, for providing a counter balance valve composed of a reduced number of constructional parts with reduced manufacturing costs and which is capable of being easily assembled.